An exemplary embodiment of this application relates to a development system for an electrophotographic reproducing machine. More particularly, the exemplary embodiment relates to a two component development system in which either ion or electron charged toner is dispensed onto a magnetic brush having magnetic carrier beads thereon to form a two component developer. The magnetic brush with the two component developer may be used to either directly develop an electrostatic latent image or to coat donor rolls for AC/DC generated toner cloud development of an electrostatic latent image.
One type of electrophotographic reproducing machine is a xerographic copier or printer. In a typical xerographic copier or printer, a photoreceptor surface is generally arranged to move in an endless path through the various processing stations of the xerographic process. As in most xerographic machines, a light image of an original document is projected or scanned onto a uniformly charged surface of a photoreceptor to form an electrostatic latent image thereon. Thereafter, the latent image is developed with an oppositely charged powdered developing material called toner to form a toner image corresponding to the latent image on the photoreceptor surface. When the photoreceptor surface is reusable, the toner image is then electrostatically transferred to a recording medium, such as paper, and the surface of the photoreceptor is prepared to be used once again for the reproduction of a copy of an original. The paper with the powdered toner thereon in imagewise configuration is separated from the photoreceptor and moved through a fuser to permanently fix or fuse the toner image to the paper.
Xerographic development systems normally fall into two categories; viz., those that use a combination of carrier beads and toner particles for two component developer material and those that use only toner particles for the developer material. In two component development systems, the carrier beads are usually magnetic and the toner particles are usually nonmagnetic, but triboelectrically adhere to the carrier beads. The toner particles are attracted to the electrostatic latent image from the carrier beads and form a toner particle image on the photoreceptor surface. In single component development systems, the toner particles are usually triboelectrically charged and generally are required to jump a gap to develop the electrostatic latent image on an image surface. Most single component development systems cause the charged toner particles to be transported to a development zone where they are caused to form a toner cloud by the action of an AC electric field. A combination of AC and DC electrical biases attract the charged toner particles in the toner cloud to the electrostatic latent image on image surface, thereby developing the image and rendering it visible.
In the electrophotographic industry, the phenomenon of triboelectricity is widely used to charge toner particles. Triboelectric charging of the toner particles is obtained by aggressively mixing the toner particles with the larger carrier beads when a two component developer material is used or by rubbing the toner particles between a doctor blade and a donor member when a single component developer material is used.
Typically, a magnetic brush development system has a sleeve that axially rotates with fixed internal magnets that attract magnetic carrier beads thereto from a sump and transport them to a development zone adjacent a movable photoreceptor. Non-magnetic particles of toner are triboelectrically attracted to the carrier beads, and as the toner particles, hereafter called toner, enters the development zone, the toner is attracted from the carrier beads to the electrostatic latent image on the confronting surface of the photoreceptor. In this configuration, the electrostatic latent image on the photoreceptor is directly developed by the two component developer on the magnetic brush.
In the image-on-image process, development of full color or multicolor electrostatic latent images requires non-interactive development systems to prevent the disturbance and contamination of previously developed image portions. Generally, full color electrostatic latent images are generally composed of a set of scanned images serially superimposed on top of each other. Each of the scanned images represent one color of the multicolor original document. Usually the magenta image portion of the latent image is developed first, followed by a yellow portion, then cyan, and finally black. Clearly, the first developed image must not be disturbed by the subsequently developed image nor must there be cross contamination of the toner images.
The type of development systems which do not disturb or cross contaminate the images as they are separately developed are referred to as non-interactive development devices and primarily relate to various powder cloud development systems. There are a number of well known non-interactive development systems, such as, for example, the scavengeless development devices as disclosed in U.S. Pat. No. 4,868,600 and U.S. Pat. No. 5,504,563. Some scavengeless development systems require stationary wire electrodes located in the toner clouds, while others types require interdigitated electrodes on donor rolls addressed by a commutator.
As mentioned above, one type of single component development is referred to as jumping development. Jumping development systems attract triboelectrically charged toner from a sump onto an axially rotated donor roll which rotates the charged toner to a location spaced from but adjacent a electrostatic latent image on a moving photoreceptor. The toner is attracted from the donor roll to the electrostatic latent image by a combination of AC and DC electric fields applied across the space or gap. Such commercial development systems as magnetic brush or jumping single component development systems with an AC electric field may interact with the photoreceptor and a previously toned image will be scavenged by subsequent development.
There are many existing scavengeless development systems that prevent interaction of the development system with the previously developed image. For example, U.S. Pat. No. 4,868,600 discloses a scavengeless development system in which toner detachment from a donor roll and the concomitant generation of a toner cloud is obtained by AC electric fields supplied by spaced wire electrodes positioned in close proximity to the donor roll and within the space between the donor roll and the photoreceptor surface containing the electrostatic latent image. In another example, U.S. Pat. No. 5,276,488 discloses a scavengeless development system in which toner is detached from a donor belt and attracted to an electrostatic latent image carried by a moving photoreceptor positioned adjacent the belt. Generation of a toner cloud is effected using AC electric fields created by applying an AC voltage between an embedded interdigitated electrode structure and a shoe stationarily positioned behind the donor belt, while U.S. Pat. No. 5,504,563 discloses a scavengeless or non-interactive development system in which an AC bias is applied between neighboring interdigitated electrodes embedded in a rotating donor roll or belt.
U.S. Pat. No. 5,656,409 discloses a method of applying non-magnetic and non-conductive toner to a rotating image containing cylinder having an electrostatic pattern thereon. The toner is contained in a container where it is fluidized and then charged by using electrically biased rotating paddle wheels to stir and charge the fluidized toner. The charged toner is transferred from the container to the rotating image containing cylinder by biased rotating cylinders.
U.S. Pat. No. 5,887,233 discloses several embodiments of devices that charge a toner layer in a single component development system. Each embodiment contains an electrification control member interposed between a charge imparting member and toner layer on a carrying roll.
U.S. Pat. No. 5,899,608 discloses a single component development system for a xerographic copier or printer having a rotatable donor roll with interdigitated electrodes. A portion of the donor roll is positioned adjacent a supply of fluidized toner contained in a housing and another portion of the donor roll is positioned at a development zone where it is adjacent a movable surface containing an electrostatic latent image. The electrodes on the donor roll may be biased to attract a layer of toner thereto. As the donor roll is rotated, the toner layer is charged by a corona-generating device and transported to the development zone. At the development zone, the electrodes are biased to produce a toner cloud to develop the latent image.
U.S. Pat. No. 6,208,825 discloses a single component development apparatus for developing electrostatic latent images on an image bearing surface. The apparatus includes a sump containing toner, a rotatable donor member having electrodes on the surface thereof for transporting toner through a development zone, and electrical biases for charging the toner in the sump. The electrodes on the donor member produce fringe fields for depositing toner on the donor member, while devices located in the development zone form a toner cloud to develop the latent image on the image-bearing surface. The apparatus further provides an electrostatic filtering zone located upstream from the development zone for removal of wrong-sign charged toner from the donor member.
U.S. Pat. No. 6,223,013 discloses a wireless hybrid scavengeless development system for developing a latent image recorded on an imaging surface in which a two component development system is used to place a uniform layer of toner onto a donor belt or roll. An electrical bias is used to load toner on the donor belt or roll. Triboelectric charging of the toner in a sump is used to assist loading of the toner onto a magnetic brush. The thickness of the toner layer on the donor belt or roll is controlled by toner concentration in the sump and an electrical bias between the donor belt or roll and the magnetic brush. Ion charging thus overwhelms the previous triboelectric charge of the toner and the donor belt or roll transports the charged toner to a development zone, whereat a toner cloud is produced to develop the latent image on the imaging member.
U.S. Pat. No. 6,377,768 discloses a development system for developing an electrostatic latent image on an image bearing surface using a movable donor roll uniformly coated with charged toner from a toner spraying device that is analogous to a powder coating mechanism. The donor roll with the toner layer is transported past a corona device to uniformly charge the toner layer and onto a development zone. The development zone is adjacent the image bearing surface where the charged toner is transferred to the latent image on the image bearing surface.
U.S. patent application Ser. No. 11/081,034 filed Mar. 16, 2005 by Dan A. Hays, SYSTEMS AND METHODS FOR ELECTRON CHARGING PARTICLES discloses systems for charging toner particles used, for example, in copying and printing machines by transporting air entrained toner particles through an electron charging device incorporating two spaced, parallel electrodes. At least one electrode is connected to an AC voltage source and at least one of the electrodes is coated with or comprised of nanotubes oriented perpendicular to the direction of entrained toner particles.
The problem with triboelectric charging of toner, as used in the known prior art, is that it causes high adhesion that limits efficient xerographic image development and electrostatic transfer of the developed image from the photoreceptor to the recording medium, such as paper. In addition, triboelectric charging toner with carrier beads requires aggressive mixing to achieve adequate charging and high shear forces are generated during the mixing and subsequent metering of the two component developer onto the magnetic brush. The high shear forces cause toner fragmentation or attrition as well as embedding of toner surface additives into the toner particles that leads to degradation in the development system performance. To achieve adequate triboelectric charging, surface additives are necessary and such additives cause the toner to be further impacted or affected by the relative humidity of the operating environment.